Horizontal nanosheet (hNS) field effect transistors (FETs) having InGaAs channels offer the possibility of high mobility, high injection velocity, and low gate capacitance. However, InGaAs hNS FETs also tend to have small bandgaps, such as approximately 750 meV or even less than 1.1 eV for silicon. A consequence of this small direct bandgap is a large amount of band-to-band tunneling (BTBT) leakage current, which is further exacerbated by the parasitic bipolar effect (PBE) that is present in all gate-all-around (GAA) FETs. Parasitic leakage, which is a combination of the BTBT leakage and PBE gain, is exponentially sensitive to positive supply voltage (VDD) and channel length. BTBT occurs in regions of strong band-bending, which exists in the transition region between the channel and the high-doped portion of the drain extension. In related art hNS FETs in which the effective channel length (Leff) is approximately equal to the gate length (LG), the hNS FETs have very high BTBT leakage and are unusable for a mobile system on a chip (SOC) operating at supply voltages of 0.75V or higher. Additionally, the channel length sensitivity presents a significant scaling limitation because hNS FETs with an Leff of approximately 15 nm may not be able to operate with VDD above approximately 0.7V without incurring excessive BTBT leakage.
In the related art, this significant scaling limitation may be addressed by configuring the FET such that the Leff is longer than the LG. Increasing Leff by shifting the PN junction away from the gate edges reduces band curvature and exponentially reduces BTBT leakage. Increasing the Leff also reduced PBE gain, which has a near-exponential sensitivity to channel length. The Leff could be increased, for instance, up to two spacer thicknesses larger than the LG. However, in related art hNS FETs, increasing Leff reduces the electron concentration in the extension regions and thus increases parasitic resistance (Rpara). The resulting increase in Rpara may render the hNS FET unsuitable. Accordingly, in related art hNS FETs, there is a tradeoff between BTBT leakage and Rpara (e.g., the BTBT leakage of related art hNS FETs may be reduced by increasing the Leff, but this reduction in BTBT leakage is at the expense of increasing the Rpara of the hNS FET).